The present invention relates to circuit breakers which suppress switching overvoltages. More particularly the present invention relates to those circuit breakers which employ resistors to suppress overvoltages.
In transmission systems of 500 KV or over, resistance closing systems have been employed in circuit breakers wherein a resistor is connected in parallel with main contacts of the breaker before the main contacts are closed. This suppresses switching overvoltages or, more particularly, closing overvoltages when the circuit breaker is closed. Typically, a linkage coupled mechanically to the main contact is generally employed for opening and closing the contact which connects and disconnects the closing resistor. Circuit breakers including the closing resistor, and particularly those with SF.sub.6 gas spraying systems, exhibit remarkably improved performance. Furthermore, it has been possible to reduce the number of breaking points. In a 500 KV system, circuit breakers with two break points have replaced circuit breakers with four break points.
However, it is necessary to consider the lowest breakdown voltage of the transmission system, the system voltage, the system surge impedance and the length of the transmission line, all of which are independent of short-circuit capacity, when determining the value of the closing resistor. Also, the heat that is dissipated during closing must also be considered. Therefore, difficulties associated with the closing resistor remain unchanged despite improvements in performance of the circuit breaker and the sharp decrease in the number of breaking points.
As the number of breaking points decreases sharply, the number of closing resistance switching contacts also decreases, reducing sharply the space needed for the circuit breaker, thus concentrating the heat that must be dissipated by the closing resistor. Consider, for example, U.S. Pat. No. 4,009,458 and Japanese Published Application No. 54-36737. These documents relate to circuit breakers in which a parallel closing resistance is employed. However, the resistance is concentrated in the contact structure associated with the resistance. Since the resistance is located in the contact, its value and capacity must be selected carefully in view of the heat that the resistor must dissipate.Also, since the heat must be dissipated from a relatively small area, miniaturization becomes difficult if not impossible. As a result, the efficient configuration of the closing resistor has become a serious matter.